A diesel-electric locomotive is not sufficient for pulling very heavy trains, in particular long goods trains. It is necessary to use two or even more such locomotives for a train. These locomotives then form a traction grouping.
Hitherto, apart from the mechanical coupling which was of course necessary, it was customary simply to connect the locomotives to one another by a control line. This control line had the purpose of controlling the additionally coupled locomotives from the driver's cab of the first locomotive. Only one locomotive driver was therefore needed.
If there was no defect in one of the locomotives, hitherto all of the locomotives of the traction grouping were operated with the same power which was predefined by the locomotive driver. If a defect occurred on a locomotive, the traction grouping immediately lost power.